A light emitting diode (LED) is a semiconductor element for converting electric energy into light. The LED has advantages of low power consumption, a semi-permanent span of life, a rapid response speed, safety and an environment-friendliness. Therefore, many researches are devoted to substitution of the existing light sources with the LED. The LED is now being increasingly used as a light source for lighting devices, for example, various lamps used interiorly and exteriorly, a liquid crystal display device, an electric sign and a street lamp and the like.